1. Field of the Invention
The present invention relates to a method of driving a simple matrix liquid crystal display panel using an STN liquid crystal or the like, and more particularly to a driving method for a liquid crystal display panel, which is suitable for half tone display by frame modulation and pulse width modulation, and is capable of driving with a low consumption power.
2. Description of the Prior Art
A simple matrix liquid crystal display panel is constructed by holding a liquid crystal layer between a row electrode group and a column electrode group to provide pixels arranged in matrix. As a method of driving the simple matrix liquid crystal display panel, there are exemplified a voltage averaging method, an SA method, an MLA method, and the like.
The voltage averaging method is a method of driving a simple matrix liquid crystal display panel, in which the respective row electrodes are sequentially selected one by one and a data signal corresponding to ON/OFF is provided to all the column electrodes in response to a selected timing. Thus, a voltage applied to each pixel becomes an applied voltage, which is high by one time per frame period T for which all the row electrodes are selected and becomes a predetermined bias voltage during a remaining non-selection time. According to the voltage averaging method, when a response speed of a liquid crystal material to be used is low, a change in brightness corresponding to an effective value of an applied voltage waveform during one frame period is obtained so that a suitable contrast is kept in practical use. However, when the number of divisions is increased, thereby being reduced in a frame frequency, a difference between one frame period and a response time of a liquid crystal becomes small, with the result that the liquid crystal provides a response every time a pulse is applied thereto. Accordingly, a brightness flicker, which is called a frame response phenomenon, is caused, resulting in degradation in a contrast.
The SA method is a method, which is called a smart addressing method, for driving a simple matrix liquid crystal display panel. In either of the voltage averaging method and the SA method, the respective row electrodes are sequentially selected one by one, and a data signal corresponding to ON/OFF is applied to all the column electrodes in response to selected timing. However, non-selection levels of common in adjacent frames are different from each other in the former and are identical in the latter.
The MLA method is also called a plural line simultaneous selection method. By simultaneously selecting a plurality of row electrodes, thereby increasing an apparent frequency and suppressing a frame response phenomenon which causes a problem in the voltage averaging method. In order to independently display each pixel while the plurality of row electrodes are simultaneously selected, a unique idea is employed for the MLA method. According to the idea, a set sequential scanning for applying a plurality of row signals represented by a set of orthogonal functions to a row electrode group in set sequence at every selection time, and a sum-of-products calculation between the set of orthogonal functions and a selected pixel data is sequentially conducted, and a column signal having a voltage level corresponding to the result is applied to a column electrode group during the selection time in synchronization with the set sequential scanning.
Note that the MLA method is disclosed in JP 05-100642 A, JP 06-027907 A, JP 07-072454 A, JP 07-193679 A, JP 07-199863 A, JP 07-311564 A, JP 08-184807 A, JP 08-184808, JP 2000-019482, and the like.
Next, a multi gray scale display method of the simple matrix liquid crystal display panel generally includes a pulse width modulation method and a frame modulation method. In the former, a waveform of a column electrode waveform is switched from ON to OFF in accordance with gray scale information within a period of a selected pulse, with the result that a frequency rises and the consumption power increases. The latter is technically established as a low-cost method. The frame modulation method is a method of selectively turning ON/OFF two gray scales of ON/OFF over a plurality of frames so that a time average value is utilized to provide two or more gray scales. Half tone display of the simple matrix liquid crystal display panel is realized by a combination of a driving method and a multi gray scale display method.
Here, examination is made on a consumption power of the simple matrix liquid crystal display panel as in the case where the pulse width modulation method is used for the multi gray scale display method and it is driven by the voltage averaging method or the SA method. Assume that the simple matrix liquid crystal display panel has a matrix of N-rows and M-columns.
FIG. 2 shows a column electrode waveform by the pulse width modulation, which is applied to the simple matrix liquid crystal display panel, by using a waveform region with oblique lines. In a middle gray scale level, there are an ON period and an OFF period within one selection time, and a round trip is necessarily made during one selection time. Thus, the column electrode waveform is changed two times. Accordingly, the number of changes in column electrode waveform per frame is 2N.
Then, examination will be made of a consumption power of the simple matrix liquid crystal display panel as in the case where the frame modulation method is employed for the multi gray scale display method, and it is driven by the voltage averaging method or the SA method. Note that the frame modulation method is conducted for every row or every pixel.
FIG. 3 shows an example of a 5-gray-scale frame modulation pattern, which is applied to the simple matrix liquid crystal display panel. In FIG. 3, when a gray scale level is 0, with respect to a first frame to a fourth frame, all values at intersections between the rows and the columns in the simple matrix liquid crystal display panel are indicated by 0 (OFF).
When a gray scale level is 1, 1 (ON) is provided to a pixel at an intersection between a (2n+1)the row and an odd column of the first frame of the simple matrix liquid crystal display panel; a pixel at an intersection between a (2n+1)the row and an even column of the second frame; a pixel at an intersection between a (2n+2)the row and an odd column of the third frame; and a pixel at an intersection between a (2n+2)the row and an even column of the fourth frame; and 0 (OFF) is provided to other pixels. In this case, n is an integer from 0 to N/2. Thus, the (2n+1)the row indicates an odd row and the (2n+2)the row indicates an adjacent even row.
When a gray scale level is 2, 1 (ON) is provided to a pixel at an intersection between the (2n+1)the row and the odd column of the first frame of the simple matrix liquid crystal display panel; a pixel at an intersection between the (2n+2)the row and an even column of the first frame; a pixel at an intersection between the (2n+1)the row and the even column of the second frame; a pixel at an intersection between the (2n+1)the row and the odd column of the third frame; the (2n+2)the row and an even column of the third frame; a pixel at an intersection between the (2n+1)the row and an odd column of the fourth frame; and a pixel at an intersection between the (2n+2)the row and the even column of the fourth frame; and 0 (OFF) is provided to other pixels.
When a gray scale level is 3, 0 (OFF) is provided to a pixel at an intersection between the (2n+1)the row and the odd column of the first frame of the simple matrix liquid crystal display panel; a pixel at an intersection between the (2n+1)the row and the even column of the second frame; a pixel at an intersection between the (2n+2)the row and the odd column of the third frame; and a pixel at an intersection between the (2n+2)the row and the even column of the fourth frame; and 1 (ON) is provided to other pixels.
When a gray scale level is 4, with respect to the first frame to the fourth frame, 1 (ON) is provided to all the pixels at intersections between the rows and the columns in the simple matrix liquid crystal display panel.
First, when the frame modulation method based on the 5-gray-scale frame modulation pattern shown in FIG. 3 is applied to the simple matrix liquid crystal display panel driven by the voltage averaging method or the SA method, to thereby conduct the multi gray scale display, the column electrode waveform in the case where an image region is scanned from the top to bottom becomes a waveform as shown in FIG. 4A or 4B. Note that, for simple description, assume that a displayed data is a data of one half tone color.
In other words, FIG. 4A shows a column electrode waveform in the case where both a pixel at an intersection between a column electrode and the (2n+1)the row electrode and a pixel at an intersection between the column electrode and the (2n+2)the row electrode are ON or OFF in the 5-gray-scale frame modulation pattern shown in FIG. 3, by using a waveform region with oblique lines. The level of the column electrode waveform in this case is +1/√N during a selection time t of one frame period T and −1/√N during a remaining non-selection time (T−t). In a next frame, the level is inverted and the similar column electrode waveform is indicated. Thus, when both upper and lower rows are turned ON or OFF in a middle gray scale level, the number of changes in column electrode waveform during one frame period is 1.
Also, FIG. 4B shows a column electrode waveform in the case where one of a pixel at an intersection between a column electrode and the (2n+1)the row electrode and a pixel at an intersection between the column electrode and the (2n+2)the row electrode is ON and the other is OFF in the 5-gray-scale frame modulation pattern shown in FIG. 3, by using a waveform region with oblique lines. The level of the column electrode waveform in this case is +1/√N during the selection time t of one frame period T. With respect to the remaining non-selection time (T−t), a level is −1/√N during a first t and +1/√N during a next t. Hereinafter, a level is similarly changed up to a final t. In a next frame, the level is inverted and the similar column electrode waveform is indicated. Thus, when rows are turned ON or OFF every two rows in the middle gray scale level, the number of changes in column electrode waveform during one frame period is N, which is equal to the number of row electrodes.
Incidentally, the consumption power of the liquid crystal panel is determined by a free discharge current between the row electrode and the column electrode. In other words, the consumption power of the liquid crystal panel is determined by a voltage value and its waveform (the amount of change) between the row electrode and the column electrode.
Therefore, in the simple matrix liquid crystal panel for performing multi gray scale display, the number of changes in column electrode waveform in the pulse width modulation method becomes larger than that in the frame modulation method, and the consumption power is also increased in proportion to the number of changes. However, in the case of the frame modulation method, the number of frames for representing gray scales is increased with increasing the number of gray scales. Thus, a flicker, roughness, or the like is caused on a screen, thereby deteriorating a display quality.